The god Thor, in desperate need of humility, was punished by his father,
Big Daddy Odin, and forced (at some undefined point in the 20th
Century) to take on the mortal identity of lame Don Blake, physician and
healer.

Before that happened, however, Thor had a number of adventures on Earth.
At one point (as shown in Thor Annual #8) Thor and Loki accidentally
traveled back in time, to 1193 B.C., and helped the Trojans and the Greeks,
respectively, during the Trojan War. During the 9th Century
C.E. Thor visited his Baltic/Norse worshipers, granting them favors and
stirring them on to further glorious feats; around the turn of the millennium
he visted them again, but on seeing a group of Vikings slaughtering a monastery
he decided to withdraw from the Earth dimension and let his worship die
out.

Not long after that (as shown in Thor Annual #7) he left Asgard
in a snit and traveled to Earth, where he encountered the Eternals
Ajak, Druig, Valkin, and Virako and helped them defeat a Deviant plot;
during this time on Earth Thor had certain adventures in North America
which have only been alluded to, never explicitly told. Around 1000 A.D.
(per Thor #401) he joined up with a group of Vikings setting out
for the New World and with them explored an ancient Atlantean outpost on
Greenland. Sometime after that--and this may be one of the adventures that
have only been alluded to--Thor visited a Viking colony in North America
that was being victimized by Varnae, the Lord of the Vampires, but was
unable to stop Varnae from wiping out the entire colony. (This was shown
in Marvel Comics Presents #63.)

And at or around 1875 Thor journeyed to the Old West to recover the
stolen Apples of Idunn (this from Thor #370). And, as Vandaljack,
Donald Campbell, and a few others noted, Thor was summoned to Germany in
1942 by Adolf Hitler and Dr. Svenson to assassinate Josef Stalin, something
which brought him into conflict with the Invaders.